The present invention relates to an objective lens and an optical head for an optical disc drive capable of recording and/or reproducing data to/from various types of optical discs having different characteristics, such as a thickness of a protective layer and a data recording density.
There are a plurality of standards regarding the characteristics of the optical discs, including the thickness of the protective layer which covers a data recording surface of the optical disc and/or the data recording density. For example, the thickness of the protective layer of a CD (Compact Disc) or a CD-R (CD recordable) whose recording density is relatively low is 1.2 mm, while that of a DVD (Digital Versatile Disc) whose recording density is relatively high is 0.60 mm.
For recording and/or reproducing data to/from the DVD, since it has a relatively high data recording density, in order to make the size of a beam spot sufficiently small, a laser beam whose wavelength is in a range of approximately 635-660 nm is to be used. For the CD-R, in view of its reflection characteristics, a laser beam whose wavelength is approximately 780 nm is to be used.
The above-described optical discs may preferably be used in a single optical disc drive. In order to allow a single optical disc drive to use both the DVD and the CD-R, the disc drive is required to have at least two laser sources respectively emitting the laser beams as described above.
In the meantime, in view of downsizing of the disc drive, it is preferable that the optical system adopted in an optical head for the disc drive is as compact as possible. In particular, optical elements such as an objective lens is preferably used for both laser beams, which enables the laser sources (i.e., laser diodes) to be implemented in a single package and provided as a light source module. An example of such an optical disc drive is described in Japanese Patent Provisional Publication No. HEI 10-261240.
If the two laser diodes are incorporated in a single package, beam emitting points of the two laser diodes are aligned in a direction perpendicular to an optical axis of the objective lens. Therefore, at least one of the beam emitting points is located off the optical axis.
According to an embodiment described in the above-described publication, a semiconductor laser chip (wavelength: 660 nm) for a DVD, whose allowable aberration is relatively small, is positioned on an optical axis of a lens system including an objective lens, a collimating lens and the like. The other semiconductor laser chip (wavelength: 780 nm) for the CD is located off the optical axis of the lens system. Therefore, the optical disc drive described in the publication has a disadvantage such that aberration, in particular coma, is relatively large for the CD (or CD-R).
Another example of the optical disc drive is disclosed in Japanese Patent Provisional Publication No. HEI 10-261241. The optical disc drive disclosed in this publication includes the optical system similar to that disclosed in the aforementioned publication (i.e., HEI 10-261240), and further, a holographic optical element (HOE) is added. With this element (HOE), an optical path of the laser beam, whose beam emitting point is located off the optical axis, is deflected (bent) so that the beam is incident on an objective lens in a direction parallel to the optical axis, thereby suppressing the coma.
The HOE disclosed in the latter publication (i.e., HEI 10-261241) utilizes a zero order component of the beam for the DVD and a xe2x88x921st order diffraction component of the beam for the CD. Therefore, it is difficult to exhibit high diffraction efficiency for both wavelengths, and loss of light is relatively large. Further, since an additional element (i.e., HOE) having a special function as above is added, the manufacturing cost of the optical disc drive increases.
It is therefore an object of the invention to provide an improved objective lens and an optical head for an optical disc drive that utilizes a light source module including at least two laser diodes emitting laser beams having different wavelengths, and a single objective optical system for both wavelengths. In this optical disc drive, aberration can be well suppressed with allowing recording and reproducing of data to/from a plurality of types of optical discs to be performed, without adding a special optical element such as the HOE.
For the above object, according to an aspect of the invention, there is provided an objective lens that converges a first laser beam having a first wavelength on a first optical disc having a first protective layer, and a second laser beam having a second wavelength on a second optical disc having a second protective layer, the first wavelength being shorter than the second wavelength, data recording density of the first optical disc being greater than data recording density of the second optical disc, a thickness of the first protective layer being smaller than a thickness of the second protective layer, the objective lens consisting of a single lens element. The objective lens further includes a first surface located on a laser emitting side, the first surface being rotationally symmetrical about a first rotational axis, and a second surface located on an optical disc side, the second surface being rotationally symmetrical about a second rotational axis, the first rotational axis and the second rotational axis being deviated relative to each other. The objective lens is configured to satisfy the following condition under a hypothetical condition where the first rotational axis coincides with the second rotational axis:
xe2x88x924.0 less than SC1/SC2 less than xe2x88x920.25,
where,
SC1 represents an offence SC against sine condition at the peripheral portion of the first region when the first laser beam is converged on the first disc,
SC2 represents an offence SC against sine condition at the peripheral portion of the first region when the second laser beam is converged on the second disc.
The offence SC against sine condition is defined by the formula below:
SC=nH1/(nxe2x80x2sinUxe2x80x2)xe2x88x92f(1xe2x88x92m)
where,
n represents a refractive index on the beam incident side medium,
nxe2x80x2 represents a refractive index on the beam emerging side medium,
Uxe2x80x2 represents an angle of the emerging beam with respect to the optical axis,
m represents a paraxial magnification,
H1 represents a ray height on a principal plane, and
f represents a focal length.
If the thus configured objective lens is employed in an optical disc drive, aberration can be well suppressed with allowing recording and reproducing of data to/from a plurality of types of optical discs to be performed, without introducing an extra optical element such as the HOE.
Optionally, the first rotational axis and the second rotational axis are inclined relative to each other.
Alternatively, the first rotational axis and the second rotational axis can be parallely shifted from each other.
Still optionally, a mark indicative of a direction of deviation is formed on an outer peripheral portion of the objective lens. The mark may be printed on the outer peripheral area of the objective lens. Alternatively, if the lens is made in accordance with the injection molding process, the mark is formed by adjusting a position of a gate.
Further optionally, the objective lens may be configured such that, under a hypothetical condition where the first rotational axis coincides with the second rotational axis, the objective lens exhibits a minimum coma for a hypothetical disc that has a protective layer whose thickness is intermediate between the thickness of the first and second discs.
According to another aspect of the invention, there is provided an optical head for an optical disc drive, which is provided with a first laser diode that emits a first laser beam having a first wavelength, a second laser diode that emits a second laser beam having a second wavelength which is longer than the first wavelength, an objective lens having a first surface which is a laser diode side surface and a second surface which is an optical disc side surface, the objective lens converging the first laser beam on a first optical disc having a first protective layer to form a beam spot, the objective lens converging the second laser beam on a second optical disc having a second protective layer to form the beam spot, a thickness of the first protective layer being smaller than a thickness of the second protective layer.
The first surface is rotationally symmetrical with respect to a first rotational axis, and the second surface is also rotationally symmetrical with respect to a second rotational axis. The first rotational axis and the second rotational axis being deviated (e.g., inclined or shifted) relative to each other. A beam emitting point of the first laser diode is positioned at a first position, coma that is caused as the first laser beam is converged on the data recording surface of the first disc being minimized when the first laser beam is emitted from the first position, and a beam emitting point of the second laser diode is positioned at a second position which is different from the first position, coma that is caused as the second laser beam is converged on the data recording surface of the second disc being minimized when the second laser beam is emitted from the second position.
Optionally, the beam emitting points of the first and second laser diodes-may be located on opposite sides with respect to a reference axis, the reference axis being an optical axis of the objective lens under a hypothetical condition where the first rotational axis coincides with the second rotational axis, and the first rotational axis and the second rotational axis, which are deviated from each other, are included in a reference plane, the reference plane being a plane including the beam emitting points of the first and second laser diodes and the reference axis.
Still optionally, the first and second positions are determined such that, by inclining the first rotational axis relative to the second rotational axis, the first laser beam is converged on a side where a thickness of the objective lens decreases and the second laser beam is converged on a side where a thickness of the objective lens increases.
Furthermore, the objective lens may be configured such that, under a hypothetical condition where the first rotational axis coincides with the second rotational axis, the objective lens exhibits a minimum coma for a hypothetical disc that has a protective layer whose thickness is intermediate between the thickness of the first and second discs.